1. Implantable Devices
Improper operation of the heart can often be remedied by the use of an implantable device, e.g., a pacemaker. These devices generally provide an electrical pulse to a selected area of the heart that is not (in terms of timing or in terms of strength) properly receiving its natural pulse. Using implantable devices, physicians have been able to provide electronic assistance for many different disorders, including pathological bradycardia (abnormal, slow heartbeat), tachyarrhythmia (abnormal, fast heartbeat), and other conditions that can, over time, pose a threat to a patient's life.
Implantable devices generally include three basic elements, i.e., one or more electrodes, a corresponding number of insulated leads, and a control unit.
The one or more electrodes are used to provide electrical stimuli directly to the heart muscle. These stimuli can be pacing pulses, and sometimes can include relatively larger shocks, such as are used to break up tachyarrhythmias. The electrodes are generally threaded directly into the heart muscle and they also can be used to electrically sense heartbeat. In addition to these electrodes, other electrical sensors may be used by the device to sense blood gases, respiration, cardiovolume, temperature, pressure or other physiological conditions.
The insulated leads are used to connect these electrodes with the control unit. The control unit is a relatively large object, and thus must be placed within the chest cavity but away from the heart. The insulated leads are laid within the chest during surgical implantation, and are also used to connect the control unit to the various other sensors, for example, pulmonary sensors at the lungs.
Finally, the control unit includes electrical circuits that generate the pacing pulses from one or more batteries. Modern day devices are sophisticated and include control logic, timing circuits, and input/output ("I/O") circuitry that interfaces the control logic with the electrodes and/or other sensors. For example, the I/O circuitry provides analog-to-digital and digital-to-analog conversion, and it generates the desired electrical stimuli as pulses of the desired strength, duration and frequency. Modern control units typically include a microprocessor and memory, and also are configured to allow remote programming after implantation in the patient's body.
2. Automaticity
Early pacemakers were fixed-rate devices, which provided electrical stimuli to the heart if it failed to beat within a predetermined time period. However, microprocessor-based technology has enabled implantable devices to make complex logical decisions based on a variety of physiological inputs. As examples, modern day implantable devices have the ability to distinguish different types of tachyarrhythmias and to select appropriate therapy that does not impose unneeded trauma on the heart. For example, for some patients, a heartbeat rate of 160 beats per minute might be considered abnormally fast, but it could be both improper and dangerous to associate that condition with a fibrillation condition, which can be treated by applying defibrillation shocks in excess of 500 volts. Such a heartbeat rate could be properly caused by physiological factors, such as stress or exercise. The present day microprocessor-based devices are capable of distinguishing normal physiological conditions from pathological conditions and also of selecting between alternative therapies for the latter. Logical decisions based on physiological variables, choice of therapies responsive to different heart conditions, and automatic self-configuration are examples of what is referred to as automaticity. For specific examples of automaticity, reference is made to a copending and commonly assigned U.S. Pat. No. 5,476,485, filed in the names of Lisa P. Weinberg and Samuel M. Katz on Sep. 21, 1993, and entitled "Automatic Implantable Pulse Generator." That application is incorporated by reference, as though fully set forth herein.
3. Shortcomings
The microprocessor-based implantable devices have proven to be of great practical utility, because they do not impose unneeded trauma on the heart and provide therapy only as needed. However, these devices do occasionally suffer from malfunction and error, albeit this is infrequent and occurs no more frequently than with other modern electronic or microprocessor-based devices. Unfortunately, however, malfunction or error is of significantly greater concern in an implantable device, because a person's life may depend on the device's proper operation. In these devices, errors could be caused by a malfunction in the hardware (the electronics) or by deterioration in the software, which might occur over time.
To minimize the possibility of these errors, many modern implantable devices are provided with fixed-rate circuits implemented in hardware, for use as a backup circuit instead of the microprocessor.. These fixed-rate circuits are triggered upon detection of parity error, and they disable all of the programmable functions of the device, which simply becomes a fixed-rate pacemaker. In other words, in response to detected error, these implantable devices presume that their decision-making circuits are in error and revert to providing a periodic pulses to the heart, losing any automaticity functions that they might have had. Often, the microprocessor is completely shut-down to conserve power, or its operation is ignored and presumed by the circuit to be faulty. An audible alarm within the pulse generator is sometimes sounded, to indicate that the patient should return to a physician's office.
However, under circumstances of software error, it is considered a waste of resources and dangerous to shut-down all of the device's automaticity functions and instead supply fixed-rate pacing to the heart, particularly because the device might be used for tachyarrhythmia or bradycardia detection and therapy, which cannot always be adequately treated by fixed-rate pacing. Otherwise stated, the very reason that the device was implanted in the patient in the first place might have been to address a condition that absolutely requires automaticity.
A further problem in using fixed-rate backup pacing is that the voltage provided to the heart might be set inappropriately high for the particular patient, and therefore might actually interfere with the heart's normal operation. Unfortunately, in an observance of caution, that is exactly how some of today's programmable pacemakers operate. The fact that a software error has been detected does not necessarily mean that the microprocessor-based system is incapable of monitoring the heart's natural operation, of providing automaticity, or of diagnosing and handling potentially dangerous conditions, such as tachyarrhythmia and bradycardia.
Accordingly, there is a definite need for an implantable device that does not shut-down all control and automaticity functions upon detection of an error in operation. Further, there is a definite need to provide these devices with an alternative backup operation that does not impose unneeded trauma on the heart. Further, such a device should be capable of determining the extent of its detected malfunction and whether or not it can continue to operate, implementing fixed-rate pacing either not at all, or only as a last resort. The present invention, as described below, provides such a device.